The presence of an aerosol or cloud (3) of a toxic substance is detected
by producing an aerosol (17, 5) of ligands (13) to the target substance
in a region of sky that contains the target substance, permitting
molecules the target substance to bind to the ligand, directing light (7,
21) of a first frequency into the revised aerosol; and then inspecting
(9, 25) the revised aerosol for emissions of light of a second frequency.
When light of the second frequency exists an alarm (15, 31) is initiated.